For sheet fabrications in gas turbine engines a variety of commercial alloys are available. These alloys can be divided into different families based on their key properties. Note that the following discussion relates to alloys which are cold fabricable/weldable, meaning that they can be produced as cold rolled sheet, cold formed into a fabricated part, and welded.
Gamma-Prime Formers.
These include R-41 alloy, Waspaloy alloy, 282® alloy, 263 alloy, and others. These alloys are characterized by their high creep-rupture strength. However, the maximum use temperatures of these alloys are limited by the gamma-prime solvus temperature and are generally not used above 1600-1700° F. (871 to 927° C.). Furthermore, while the oxidation resistance of these alloys is quite good in the use temperature range, at higher temperatures it is less so.
Alumina-Formers.
These include 214® alloy and HR-224® alloy, but not the ODS alloys (which do not have the requisite fabricability). The alloys in this family have excellent oxidation resistance at temperatures as high as 2100° F. (1149° C.). However, their use in structural components is limited due to poor creep strength at temperatures above around 1600-1700° F. (871 to 927° C.). Note that these alloys will also form the strengthening gamma-prime, but this phase is not stable in the higher temperature range.
Solid-Solution Strengthened Alloys.
These include 230® alloy, HASTELLOY® X alloy, 617 alloy, and others. As their name implies, these alloys derive their high creep-rupture strength primarily from the solid-solution strengthening effect, as well carbide formation. This strengthening remains effective even at very high temperatures—well above the maximum temperature of the gamma-prime formers, for example. Most of the solid-solution strengthened alloys have very good oxidation resistance due to the formation of a protective chromia scale. However, their oxidation resistance is not comparable to the alumina-formers, particularly at the very high temperatures, such as 2100° F. (1149° C.).
Nitride Dispersion Strengthened Alloys.
These include NS-163® alloy which has very high creep-rupture strength at temperatures as high as 2100° F. (1149° C.). While the creep-rupture strength of NS-163 alloy is better than the solid-solution alloys, its oxidation resistance is only similar. It does not have the excellent oxidation resistance of the alumina-formers.
What is clear from the above discussion is that there is no cold fabricable/weldable alloy commercially available which combines both high creep-rupture strength and excellent oxidation resistance. However, in the effort to continually push gas turbine engine operating temperatures higher and higher, it is clear that alloys which combine these qualities would be very desirable.